(1) Field of the Invention
The present invention relates generally to a system and method for improving the efficiency of combustion reactions and for reducing pollutants, and, more particularly, to a system and method for improving combustion efficiency and reduction of nitrogen oxides (NOx) using nitrogenous agents.
(2) Description of the Prior Art
Increases in fuel costs have required power generation plants seek increases in furnace efficiencies in order to reduce power generation costs. However, NOx formation must also be prevented to comply with environmental regulations. NOx formation is reduced in furnaces by the process of stage combustion, which includes administering an initial substoichiometric or suboptimal ratio of oxygen to fuel to maintain combustion gas temperatures below the peak NOx-producing temperature, about 2,800 degrees F. (approximately 1540 degrees C.), followed by the addition of secondary air, or over-fire-air (OFA), to finish the combustion reaction. Proper mixing of secondary air and combustion gases inside a furnace is thus important to achieve optimum combustion and has been improved by the use of rotating over-fire-air (ROFA). However, these existing NOx reduction systems do not optimize combustion efficiency or furnace heat exchange efficiency.
Therefore, a need exists to improve energy efficiency of ROFA systems without negatively affecting, or even improving the reduction of pollutants, in particular NOx reduction.
The use of urea and NH3-generating compounds is known in the prior art. Example of the use of urea and NH3-generating compounds include U.S. Pat. No. 4,992,249 issued Feb. 12, 1991 to Bowers for Reduction of nitrogen- and carbon-based pollutants through the use of urea solutions and U.S. Pat. No. 4,927,612 issued May 22, 1990 invented by Bowers for Reduction of nitrogen- and carbon-based pollutants teaches process using a dispersion of aqueous urea solution is injected into an effluent for reducing nitrogen oxides in an effluent from the combustion of carbonaceous fuel.
U.S. Pat. No. 5,057,293 issued May 22, 1990 invented by Epperly, et al. and assigned to Fuel Tech, Inc. for Multi-stage process for reducing the concentration of pollutants in an effluent teaches a process for the reduction of the concentration of nitrogen oxides in the effluent from the combustion of a carbonaceous fuel, the process comprising selecting a plurality of locations for introduction of chemical formulations and introducing at each of said locations at least one chemical formulation, selected from the group consisting of urea, ammonia, hexamethylenetetraamine, an oxygenated hydrocarbon, a paraffinic hydrocarbon, an olefinic hydrocarbon, an aromatic hydrocarbon, an ammonium salt of an organic acid having a carbon to nitrogen ratio of greater than 1:1, a hydroxy amino hydrocarbon, a heterocyclic hydrocarbon having at least one cyclic oxygen, a five- or six-membered heterocyclic hydrocarbon having at least one cyclic nitrogen, hydrogen peroxide, guanidine, guanidine carbonate, biguanidine, guanylurea sulfate, melamine, dicyandiamide, calcium cyanamide, biuret, 1,1′-azobisformamide, methylol urea, methylol urea-urea condensation product, dimethylol urea, methyl urea, methyl urea, and mixtures thereof, effective to reduce the concentration of nitrogen oxides at the effluent temperature existing at said location, such that optimization of the level of injection at each of said locations leads to the reduction of the level of nitrogen oxides below a predetermined target level.
U.S. Pat. No. 4,208,386 issued Jun. 17, 1980 to Arand, et al. for Urea reduction of NOx in combustion effluents and U.S. Pat. No. 4,325,924 issued to Arand, et al. on Apr. 20, 1982 for Urea reduction of NO.sub.x in fuel rich combustion effluents teach methods for reducing NOx in combustion effluents involving introducing urea into the combustion effluent.